The study of the temperate bacteriophage has been exceedingly fruitful in generating information about universal regulatory mechanisms. Control of gene expression by suppression or induction of transcription termination, first described in phage, is now known to be important in eukaryotic cells, in particular in the control of c-myc and of HIV gene expression. In lambda, antitermination by the N protein allows phage growth, whereas termination induced by the Nun protein of the rival phage, HK022 blocks lambda development. The 109 aa Nun protein is a member of the arginine-rich RNA binding protein ARM family that includes N, HIV-1 Tat, and HIV-1 Rev proteins. This proposal is to study the mechanism of action of Nun through biochemical and genetic approaches. The structures of Nun complexes with RNA, the host NusA factor, or RNA polymerase will be probed by NMR, fluorescence ainsotropy and crosslinking strategies. A possible interaction between the C-terminus of Nun and the DNA template is to be explored. The C-terminus of Nun carries a novel Zn2+ binding motif that is essential for termination; the role of this ligand in Nun-promoted reactions is to be studied. Chemical studies will be performed in concert with mutagenesis of Nun and host proteins to define interacting residues. Host factors that play a role in Nun termination and cell killing are to be defined genetically and characterized b2iochemically. These include NusG, a putative factor that releases Nun-arrested transcription elongation complexes, and a factor that allows host killing by the Nun protein.